Graphite is a highly valuable industrial commodity, by virtue of a unique combination of physical and chemical properties. Graphite has a remarkable resistance to high temperatures (when maintained in a non-oxidizing atmosphere), and resistance to thermal shock, together with a high resistance to flow therethrough of electrical current, Consequently, it is ideally suited for the formation of industrial electrodes, especially for steel making furnaces and the like, which can be of very large size, e. g. up to 12' or more in length and 2 to 3' or greater in diameter, as well as for crucibles, molds, and the like for receiving and/or shaping molten metals. It is essentially inert to chemical or corrosive attack and is thus peculiarly well adapted as a structural material for chemical reactors, as an electrode in the electrolytic production of various chemicals, such as chlorine, caustic soda and so on, and metals, such as magnesium and titanium. It is also an effective absorber of nuclear radiation and hence finds extensive application as a construction and shielding material for nuclear reactors.
As is apparent, many of these applications entail exposure of the graphite to more or less high temperature and therefore, the thermal expansion properties of graphite becomes a significant, or even critical, property, depending, of course, on the specific application. In most situations, the graphite will be in the form of bodies of considerable size and thermal expansion will take place in two perpendicular directions, i. e. axially and transversely, so that the relative extent of the thermal expansion in these two directions, i. e. the ratio of axial to transverse expansion or vice versa, becomes a determining factor in the suitability of a particular graphite for a particular end use.
As with various other materials, the thermal expansion of graphite is measured in terms of a "coefficient of thermal expansion" (CTE), i. e. the change in length per unit length per degree of temperature variation, often reported as 10.sup.-7 in/in/ .degree.F., and from such values, the ratio can be determined. Crystalline graphite is constituted of carbon atoms bonded trigonally and arranged in planar sheets which are stacked perpendicular to the planes of the sheets. For a true crystalline graphite, the CTE in a direction parallel to these planes (which is considered to be the "axial" direction) is quite low, perhaps as low as 0.1 whereas the CTE in a direction normal to the planes (which is considered the "transverse" direction), is quite perhaps as high as or higher than 20. Such as material is obviously highly "anisotropic". Fur some purposes, for example, for making electrodes for steel production, where the graphite body is essentially elongated, high anisotropicity can be tolerated or even desirable.
On the other hand, where the primary function of the graphite in a given end use is structural, e. g. in a nuclear reactors, it is usually desirable for the graphite to exhibit generally the same CTE in both directions so that any expansion with temperature is about the same in both directions, minimizing the risk of cracking or other loss of structural integrity due to uneven expansion. Such a graphite is referred to as "isotropic". In still other cases of end uses, for example in electrodes for electrolytic refining of aluminum, an intermediate degree of anisotropicity is preferable.
For an extensive discussion of the production of graphite particularly for electrode use with full details of the techniques and procedures employed for the various steps involved, reference may be had to the article "GRAPHITE ELECTRODES--A Staff-Industry Collaborative Report", Industrial and Engineering Chemistry, Vol 46, No. 1, January, 1954, pp. 2-11, which is incorporated by reference herein.
Although strictly speaking, it is the ratio of CTE values that determines isotropicity or anisotropicity, as a general rule in the graphite industry, a relatively high CTE value in the axial direction, e. g. about 4 or higher, is taken as a matter of convenience as an indication of a substantially isotropic material. Conversely, an axial CTE value of about 1 or below is taken as an indication of a relatively highly anisotropic graphite. While different graphites can exhibit variation in the degree of transverse thermal expansion, that variation is presumably significantly less than the variation in axial expansion so that this approximation is a useful guideline.
Natural graphite is available in ample quantities but is rarely because it contains significant amounts of impurities and its purification is expensive. Instead, virtually all "modern" graphite is so-called synthetic or artificial graphite. Although coal, especially anthracite coal, in the beginning served as the primary raw material for making graphite, for at least the last 75 years or so, the primary raw material for synthetic graphite in the United States has been petroleum coke, i. e. a coke residue from the refining by destructive distillation or cracking of petroleum into lighter fractions, such as gasoline, etc. Being essentially a by-product in a process aimed at other higher value products, the quality of the resultant coke has not been a major concern to the petroleum refining industry and the properties of petroleum coke may differ from refiner to refiner or from refining run to run. Moreover, the coke from different kinds of crude oil can vary and as natural sources of high quality ("sweet") crude oil become exhausted, compelling the refining industry turn to lower quality crude oils, the consistent availability of petroleum coke having properties desirable for graphite production is an increasing problem to the synthetic graphite industry.
In our patent application Ser. No. 08/302,481, filed Sep. 12, 1994, which is a continuation of application Ser. No. 07/949,985), filed Sep. 24, 1992, we described and claimed our unexpected discovery that a high quality strongly isotropic graphite could be produced from a coal fraction derived by solvent extraction of a bituminous coal utilizing as the extraction solvent N-methyl pyrrolidone (NMP) or an equivalent thereof by the low temperature carbonization (i. e. at around 500.degree. C.) and/or medium temperature carbonization (i. e. at around 1000.degree. C.) followed by graphitization at around 2800-3000.degree. C. or higher, all under non-oxidizing conditions. The details of the extraction procedure employing such a solvent as well as the class of solvents effective therein are set forth in U.S. Pat. No. 4,272,356 to Stiller et al to which reference may be had to additional information.
Analysis of a number of petroleum cokes suitable for graphitization which are available in the United States established that they are highly variable depending on coke type and source, with, for instance an axial CTE ranging from 0.15 to over 4.0. It is possible to produce cokes for graphitization from pitches derived from coal tars but at the present time, all available are from foreign sources, mainly Japan, Typically, these cokes have an axial CTE in the range of about 3-4 and are thus relatively isotropic in nature. Parenthetically, the isotropic/anisotropic character of a graphitizable coke essentially carries over into the ultimate graphite and thus the thermal expansion behavior as a measure of the isotropicity or anisotropicity of a given coke can be considered to be a reliable indication of the thermal expansion behavior of a graphite made from than coke. That is to say, if a calcined coke exhibits substantial isotropicity, the graphite produced from that calcined coke will exhibit substantially the same isotropicity; similarly, if a calcined coke is substantially or highly anisotropic, the graphite produced from that coke will have essentially the same degree of anisotropicity; and the same correspondence applies between these extremes.
As noted above, there is a definite commercial need for graphites having some intermediate level of anisotropicity between substantially isotropic and highly anisotropic. Up to now, the synthetic graphite industry has dealt with this need by forming mixtures in ground or powdered form of a plurality of different cokes having different thermal expansion behavior in order to achieve a suitable graphite. Necessarily, this involves a good deal of experimental trial and error before the proper or desired combination of properties in the final graphite has been reached. Furthermore, even when the coke mixture is finely ground, there is in the final graphite a perceptible non-homogeneity on a microscopic scale which is undesirable but thus far unavoidable.
If it were possible in some economically feasible manner to produce graphites of generally or substantially controlled or selective anisotropicity between the extreme limits of almost 0% anisotropicity (i. e. highly) isotropic) and highly substantially anisotropic and that are free of this microscopic non-homogeneity, this would be a valuable step forward in the graphite field.